He studied at the National College of Art (now the National College of Art and Design) in Dublin from 1950, where he received major awards for painting and composition, studying under Seán Keating, Maurice MacGonigal and Laurence Campbell.
While still a student, Walsh had a solo exhibition in 1953 at the Dublin Painter's Gallery opened by the Earl of Wicklow.
[1] In 1954 he won the first Royal Hibernian Academy Scholarship in Art, the MacAuley Fellowship, after he graduated.
[1][2] In December 1959, he founded The Independent Artists' Group (IAG)[1] in opposition to the Living Art Group and the RHA with James McKenna, Elizabeth Rivers, Noel Sheridan, Patrick Pye and Charles Brady.
[2] The Group's first exhibition took place in June 1960, and was opened by Dr Noel Browne TD.
[1] Walsh was part of a group known as "Baggatonia" alongside Brendan Behan and Patrick Kavanagh.
[1] His work is characterised by strong colour and post-impressionistic style, which was likened to Henri Matisse.